The kettlebell, known as the girya in the East, is a fitness tool supposedly originating from Russia, though historical evidence also suggests Germany. It was used to weight crops in the 18th century, one girya weighting one pood, about 16.38 kg. Modern kettlebells come in a variety of weights, but multiples of 8 seem to be the most common.
Its use in circus and strength athletics is recorded since the 19th century, mainly in Russia. However, similar weights, such as the Greek halteres, have been used in various parts of the world since Antiquity.
Unlike dumbbells, the kettlebell's ball and handle shape makes it especially suitable for balistic and swinging movements, or even juggling. Kettlebell practice is known to have many health benefits, since it puts severals muscles to work simultaneously and provides intense cardiovascular training.
I currently use a 20 kg kettlebell, which falls between the more usual weights of 16 and 24 kg. I would have ideally started with 16 kg, but it was the last one in the shop and I felt stupidly confident, so now I have no choice but to become actually stronger.